Apparatus for the electrolysis of aqueous alkali sulphate solutions



y 8 1956 w. MULLER 2,744,864

APPARATUS FOR THE ELECTROLYSIS OF AQUEOUS ALKALI SULPHATE SOLUTIONS Filed Feb. 14, 1.951

INV ENTOR. W/LHEZM MULLER wrflm TORNEYS fl w Pa ifi m? 2,7;864' Y ARAT YURI H C RO YSIS 0F AQUEOUS ALKA'LFSULPHATE SOLUTIONS Wilhelm Miiller, Darmstadt, Germany, assignor to E. Merck, Darmstadt, Germany, a company Application February 14, 1951, SerialNo. 210,87 9

1 Claim. .(Cl. 204 220) Thepres'en'tinvention relates to an improvement in the construction and working of electrolytic cells as used for the production of caustic-alkali by the electrolysis of solutions of alkali metal sulphates by the amalgam process, and is particularly suitable for the electrolysis of sodium sulphate. i

l The electrolysis of aqueous alkali sulphate solutions according to the amalgam process is generally carried out in an electrolytic cell with a mercury layer flowing over the bottom of the cell serving as cathode and with an anode placed above it. Such a process is, however, only of commercial value if the cell voltage is low, and this necessitates that the anode must be separated from the cathode by only a few millimeters. In that case, however, the sulphuric acid evolved at the anode introduces a disturbing factor so that its difiusion back to the cathode must be prevented. For this purpose a diaphragm is generally introduced between the electrodes which leads to difficulties of construction as it is diflicult to introduce suflicientlv large diaphragms into the narrow space between the electrodes while at the same time insuring that the diaphragm remains free without touching the mercury surface. If it should do so, violent decomposition of the amalgam takes place. Furthermore, it is not possible to avoid with certainty the formation of small quantities of hydrogen at the cathode. This gas collects then under the diaphragm and in time hinders the passage of the current through the cell. It has been attempted to get over this last named difficulty by inclining the diaphragm but as the mercury surface can only be very slightly inclined, this leads to the distances between the electrodes being too great to make patent specification it can be seen, however, that with the usually used current densities of to amperes per square decimeter, it is necessary to use voltages in the neighborhood of 6 to 7 volts which are too high for commercial practice.

According to the present invention the drawbacks of previous arrangements can be avoided in the electrolysis of aqueous alkali salt solutions especially alkali sulphate solutions by placing the anodes in casings of non-conducting material resistant to the action of the electrolyte, the lower ends of which are only a little Way above the mercury surface and extend to above the top surface of the electrolyte, the electrolyte being introduced in such a manner that it must rise up inside the narrow channels thus surrounding the anodes. In this way the hydrogen ions formed by dissociation of the sulphuric acid formed at the anode are prevented from wandering to the cathode. The speed of circulation necessary for obtaining this effect is very small and amounts to about 10 to 200 millimeters per hour according to the current density and the desired concentration of sulphuric acid. The anodes may advantageously be formed from continuous or often interrupted comparatively long and narrow bands.

2,744,864 l atent ed -M ay 8,-1956 A particularly convenient method or carrying out the invention consists in forming the anode in the form of a large number or rods in narrow quadrilateral channel-like containers having the required dimensions which advantageously arear'rang'edone behind the other and transverse to th'e direction of the 'flow of the mercury. ese channel-like members or casings are furnished with an overflow for carrying away the acid anolyte. They can be closed at the top in, order to recover the o'g'ry'gen formed at the anode. In this case care should be taken that "there is a good pressure equilibrium between the cathode and anode spa'eesas otherwisedis'turbances ensue in the elect'roly'sis. For this reason it is generally advantageous to cause the gas spaces over the two electrodes to communicate. This can safelyv be carried out in practice as the evolution of hydrogen Fat the cathode is only very slight.

A very advantageous and convenient method of carrying out the invention is one in which the channel-like vessels surrounding the anode are closed on their undersides with a fine sieve or ,porous fabric-or diaphragm of nonconducting material so that the back diffusion of the sulphuric acid to the cathode is still better prevented and at the same time any material falling away from the anode is caught. It is also advantageous to give this semi-permeable membrane a slight inclination to facilitate the escape of any hydrogen which is evolved. In the case of anodes arranged transversely to the direction of the mercury stream this inclination is advantageously arranged upwards in the direction of the mercury flow so that the suction effect of the moving mercury is helpful in removing any hydrogen bubbles.

A specially valuable arrangement of apparatus and method of carrying out the invention is illustrated in the accompanying drawing. Fig. 1 shows a plan of the ap paratus; Fig.2 is a cross-section of Fig. 1 through the line PP; and Fig. 3 is a longitudinal section along the line CC through the anode cells.

K is the electrolytic vessel, on the bottom of which a thin layer of mercury spread over the whole of the vessel flows from right to left whereby it becomes loaded with alkali metal. The amalgam thus formed flows under the separating wall T into the decomposition channel B, in which it is decomposed by water. The water is added through W and the alkali lye thus obtained flows away at L. Q is a pump which raises the mercury to a higher level in order to maintain the circulation in a clockwise direction as shown. The mercury passes under the separating wall Z into the front vessel F in which the electrolyte is introduced through E. The anodes in the electrolytic vessel K are in the form of narrow bands A which are housed in long rectangular casings. The mercury layer Hg flows along the bottom of the electrolytic vessel K. A few millimeters above this is the anode A in casings G formed of insulating material. The casings G are closed underneath with a fabric M formed of fibers chemically resistant to corrosion by the electrolyte. The casings G, as well asthe network M beneath the anodes A, may be formed of various materials including plastics and polyvinyl chloride has been found to be quite satisfactory for this purpose. The electrolyte which is introduced through opening E flows through the porous bottom M of the casings, rises to above the anodes A and flows from the anode casings through outlets U to the channel R, whence it escapes through outlet S.

In the apparatus described according to these drawings in working for a long period of time a solution of 350 g. of sodium sulphate per liter at 58 C. with an anode current density of 22 amperes per square decimeter and at a cell voltage of 4.70 to 4.90 volts is electrolyzed. A current yield of 98 to 99% of theory is obtained and a sulphuric acid concentration in the anolyte of g. per liter.

Having thus described the present invention so that others skilled in the art may be able to understand and practice the same, I state that what I desire to secure by Letters Patent is defined in what is claimed.

What .is claimed is:

Apparatus for the electrolysis of aqueous alkali sulphate solutions comprising a fixed electrolytic vessel having an inlet end and an outlet end, an anolyte channel at one side thereof and having an outlet for anolyte near said outlet end, a decomposition channel connected to the outlet end of said vessel, means associated with said decomposition channel for withdrawing mercury therefrom and causing it to enter the inlet end of said vessel and toflow on the bottom thereof to said outlet end, the end walls of said vessel extending into the mercury layer on the bottom wall of the vessel and sealing against escape of electrolyte therebetween, said vessel having openings in the upper part of its inlet and outlet ends for entry of electrolyte and for withdrawal of gases, respectively, a plurality of anode chambers in said vessel, each chamber comprising vertical walls defining a space and a porous diaphragm at its lower end adjacent to mercury on the bottom wall of said vessel, said walls and diaphragm being composed of non-conducting material resistant to the electrolyte, and an anode in each chamber having its vertical edges disposed close to said vertical walls and defining therewith narrow passages for the flow of gases and liquids therebetween, the bottom surface of the anode lying close to the diaphragm and both said surface and diaphragm sloping upwardly in the direction of flow of mercury on the bottom of the vessel, each anode chamber having a passage leading into the anolyte channel at a level substantially below the levelof the electrolyte in said vessel.

References Cited in the file of this patent UNITED STATES PATENTS 738,094 Brichaux Sept. 1, 1903 742,865 Hannon Nov. 3, 1903 803,263 Anderson Oct. 31, 1905 1,117,879 McDonald Nov. 17, 1914 2,150,775 Messner Mar. 14, 1939 FOREIGN PATENTS 73,526 Austria July 10, 1917 471,912 Great Britain Sept. 13, 1937 701,771 Germany Jan. 23, 1941 

